What is added to the 3' end of mRNA to enhance stability?

The addition of a Poly-A tail to the 3' end of mRNA serves a crucial function in enhancing the stability of the molecule. This repetitive sequence of adenine nucleotides is added post-transcriptionally in eukaryotic cells and plays several important roles.

First, the Poly-A tail protects mRNA from degradation by exonucleases, which are enzymes that could otherwise break down the RNA. The presence of this tail makes the mRNA more stable in the cytoplasm, extending its lifespan and allowing sufficient time for translation into proteins.

Second, the Poly-A tail is involved in the regulation of translation and nuclear export. It aids in the recruitment of cellular machinery needed for the translation process, ensuring that the mRNA can be efficiently translated into proteins.

In contrast, options like the 5' cap also play a role in mRNA stability and translation; however, it is the Poly-A tail specifically that is added to the 3' end for enhancing stability. Methyl groups and introns, while important in other contexts, do not directly relate to the stabilization of mRNA in the manner that the Poly-A tail does.